This invention relates to a polycrystalline cubic boron nitride (cBN) abrasive. Among the grinding wheels, to which the abrasive of the present invention is applied, there are, above all, a metal bonded grinding wheel, an electroplated grinding wheel and a vitrified grinding wheel for heavy loaded or high-speed grinding.
Since cBN, which is a high-pressure phase of boron nitride, has a hardness and a thermal conductivity next to diamond, and a characteristic of not being reacted with iron-based metal which is absent in diamond, it is being exploited as an abrasive for grinding the iron-based metal. The recent tendency of the grinding operation is towards labor saving and unmanned operation. To this end, heavy loaded grinding and high-speed grinding are carried out. However, under hostile grinding conditions imposed in this manner, a large load is applied to the grinding wheel, above all, on the abrasives, so that higher strength is required of the abrasive.
Among the high-strength cBN abrasives, an abrasive having a polycrystalline structure is known, which partially has been commercialized. The polycrystalline abrasive has a polycrystalline structure in which fine crystal grains are strongly bonded to one another, so that it is tough, and hence exhibits a high strength, in contrast to a monocrystalline abrasive in which one single grain is constituted by a monocrystal. The polycrystalline abrasive is produced by pulverizing a sintered body obtained by a non-catalytic direct conversion method to a desired grain size, thus in a manner different from the catalytically synthesized monocrystalline abrasive, as disclosed in U.S. Pat. No. 4,289,503. Although the polycrystalline abrasive is tougher than a monocrystalline abrasive, if the abrasives are used for a grinding wheel under hostile conditions including heavy loaded or high-speed grinding, abrasives on the surface of the grinding wheel are partially destroyed or worn out to present a coarse work surface or to lower the cutting performance, so that dressing or tooling operations need to be carried out frequently.